Cover gases

ABSTRACT

A cover gas composition for protecting molten magnesium/magnesium alloy includes a fluorine containing inhibiting agent and a carrier gas. Each component of the composition has a Global Warming Potential (GWP) (referenced to the absolute GWP for carbon dioxide at a time horizon of 100 years) of less than 5000.

FIELD OF THE INVENTION

The present invention relates to compositions useful as cover gases forprotecting molten magnesium/magnesium alloys. The present invention alsorelates to a method for protecting molten magnesium/magnesium alloys andto a method for extinguishing magnesium/magnesium alloy fires.

BACKGROUND ART

Magnesium is a highly reactive and thermodynamically unstable element.Molten magnesium is readily and violently oxidised in ambient air,burning with a flame temperature of approximately 2820° C. Threeapproaches have been used to inhibit the severe oxidation process. Saltcover fluxes may be sprinkled over the molten metal; oxygen may beexcluded from contacting the molten metal by blanketing the molten metalwith an inert gas such as helium, nitrogen or argon; or a protectivecover gas composition may be used to blanket the molten metal.Protective cover gas compositions typically comprise air and/or carbondioxide and a small amount of an inhibiting agent which reacts/interactswith the molten metal to form a film/layer on the molten metal surfacewhich protects it from oxidation. To this day, the mechanism by whichinhibiting agents protect molten reactive metals is not well understood.

U.S. Pat. No. 1,972,317 relates to methods for inhibiting the oxidationof readily oxidisable metals, including magnesium and its alloys. Thepatent notes that at the time of its filing in 1932, numerous solutionshad been proposed to the oxidation problem including displacing theatmosphere in contact with the metal with a gas such as nitrogen, carbondioxide or sulphur dioxide. U.S. Pat. No. 1,972,317 teaches inhibitionof oxidation by maintaining in the atmosphere in contact with moltenmetal an inhibiting gas containing fluorine, either in elemental orcombined form. Reference is made to many fluorine containing compoundswith the solids ammonium borofluoride, ammonium silicofluoride, ammoniumbi-fluoride and ammonium fluophosphate or the gases evolved therefromupon heating being said to be preferred. Notwithstanding the issue ofU.S. Pat. No. 1,972,317 in 1934, it was not until about the mid-1970'sthat a fluorine containing compound found commercial acceptance as aninhibiting agent in a cover gas.

Prior to about the mid-1970's, sulphur dioxide (SO₂) was widely used asan inhibiting agent in a magnesium cover gas composition but wasreplaced by sulphurhexafluoride (SF₆) which has become the industrystandard. Typically, SF₆ based cover gas compositions contain 0.2–1% byvolume SF₆ and a carrier gas such as air, carbon dioxide, argon ornitrogen. SF₆ has the advantages that it is a colourless, odourless,non-toxic gas which can be used for protecting moltenmagnesium/magnesium alloy and in the production of bright and shinyingots with relatively low dross formation. However, SF, suffers fromseveral disadvantages. Its sulphur based decomposition products at hightemperature are very toxic. It is expensive, has limited sources ofsupply, and is one of the worst known greenhouse gases having a GlobalWarming Potential (GWP) at a time horizon of 100 years of 23,900relative to 1 for carbon dioxide.

It is also noted that once magnesium has ignited, the resulting firecannot be extinguished even with high concentrations of SF₆. SO₂ is evenworse in this respect as it can accelerate a magnesium fire. The onlyknown cover gas for extinguishing a magnesium fire is boron trifluoride(BF₂) which is very expensive and very toxic.

Alternative cover gas compositions are desirable.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a cover gascomposition for protecting molten magnesium/magnesium alloy, thecomposition including a fluorine containing inhibiting agent and acarrier gas, wherein each component of the composition has a GlobalWarming Potential (GWP) (referenced to the absolute GWP for carbondioxide at a time horizon of 100 years) of less than 5000.

Preferably, the inhibiting agent has minimal ozone depletion potential,more preferably the inhibiting agent has no ozone depletion potential.

Preferably, the inhibiting agent is non-toxic. In this regard, compoundshaving a Threshold Limit Value-Time Weighted Average (TLV-TWA) (the timeweighted average concentration for a normal 8 hour workday and a 40 hourworkweek, to which nearly all workers may be repeatedly exposed, dayafter day, without adverse effect) as issued by the American Conferenceof Governmental Industrial Hygienists of less than 100 ppm areconsidered to be toxic. By way of example, BF₂, silicon tetrafluoride(SiF₄), nitrogen trifluoride (NF₃) and sulfuryl fluoride (SO₂F₂)disclosed in U.S. Pat. No. 1,972,317 are toxic.

The composition may include a mixture of inhibiting agents (each havinga GWP less than 5000) and preferably comprises a minor amount ofinhibiting agent and a major amount of a carrier gas. Preferably, thecomposition consists of less than 1% by volume inhibiting agent and thebalance carrier gas. More preferably, the composition contains less than0.5% by volume (most preferably less than 0.1% by volume) inhibitingagent.

Preferably, each component of the composition has a GWP of less than3000, more preferably, less than 1500.

Suitable carrier gases include air, carbon dioxide, argon, nitrogen andmixtures thereof.

The inhibiting agent may be selected from the group consisting ofhydrofluorocarbons (HFCs), hydrofluoroethers (HFEs) and mixturesthereof. Preferably, the inhibiting agent has a boiling point of lessthan 100° C., more preferably less than 80° C. Where the inhibitingagent is gaseous at ambient temperature, it may be diffused in thecarrier gas at the desired concentration. Where the inhibiting agent isliquid at ambient temperature, it may be entrained in the carrier gas toa desired concentration by passing a flow of carrier gas over theinhibiting agent. Suitable hydrofluorocarbons and hydrofluoroethers arelisted in Table 1 below which includes their boiling points (BP) andtheir GWP's (referenced to the absolute GWP for carbon dioxide at a timehorizon of 100 years) which have been sourced from IPCC 1996.

TABLE 1 Industry Chemical Name Name Formula GWP BP difluoromethaneHFC-32 CH₂F₂ 580 −52° C. pentafluoroethane HFC-125 C₂HF₅ 3,200 −49° C.1,1,1,2-tetrafluoroethane HFC-134a, C₂H₂F₄ 1,300 −26° C. R134adifluoroethane HFC-152a, C₂H₄F₂ 140 −27° C. R152a heptafluoropropaneHFC-227ea C₃HF₇ 2,900 −17° C. methoxy-nonafluorobutane HFE-7100 C₄F₉OCH₃480   61° C. ethoxy-nonafluorobutane HFE-7200 C₄F₉OC₂H₅ 90   78° C.dihydrodecafluoropentane HFC-43- C₅H₂F₁₀ 1,300   54° C. 10-mee

A preferred cover gas composition consists of 1,1,1,2-tetrafluoroethaneand dry air. Experimental work has demonstrated that such a cover gascomposition provides protection at least the equal of SF₆ basedcompositions and can be utilised at lower concentrations of inhibitingagent. SF₆ has a GWP in excess of 18 times that of1,1,1,2-tetrafluoroethane and is presently more than 2½ times the costof 1,1,1,2-tetrafluoroethane.

In a second aspect, the present invention provides a method ofprotecting molten magnesium/magnesium alloy, the method includingblanketing the molten magnesium/magnesium alloy with a cover gascomposition according to the first aspect of the present invention.

The method according to the second aspect of the present invention isapplicable to protecting molten magnesium/magnesium alloy in a foundryvessel such as a furnace and during casting.

In a third aspect, the present invention provides use of an inhibitingagent as defined with respect to the first aspect of the presentinvention for preventing or minimising oxidation of moltenmagnesium/magnesium alloy. By way of example, an inhibiting agent of thepresent invention may be used to prevent or minimise oxidation of moltenmagnesium/magnesium alloy during sand casting. Where the inhibitingagent is gaseous at ambient temperature, the sand mould may be purgedwith inhibiting agent prior to pouring of the molten metal. Where theinhibiting agent is liquid at ambient temperature, the sand mould may besprayed with inhibiting agent from a squeeze bottle or the like prior topouring of the molten metal. Other suitable methods of using inhibitingagents of the present invention to prevent or minimise oxidation ofmolten magnesium/magnesium alloy will be readily apparent to those ofskill in the art of foundry practice.

In a fourth aspect, the present invention provides a method ofextinguishing a magnesium/magnesium alloy fire, the method includingexposing the fire to an atmosphere of an inhibiting agent as definedwith respect to the first aspect of the present invention. The fire maybe so exposed by, for example, subjecting it to a flow of the inhibitingagent or immersing it in a reservoir containing the inhibiting agent.

EXAMPLES

The ensuing non-comparative Examples are illustrative of preferredembodiments of the present invention and are not to be construed aslimiting the scope of the present invention in any way.

Example 1

A crucible furnace containing 100 grams of molten pure magnesium at 680°C. was blanketed with a gaseous composition consisting of 0.02% byvolume 1,1,1,2-tetrafluoroethane and the balance dry air. Good moltenmagnesium protection was observed, with the formation of a thinprotective surface film. Deliberate rupturing of the surface film didnot induce burning of the molten magnesium sample.

Comparative Example 1

Comparative Example 1 was identical to Example 1 with the exception that1,1,1,2-tetrafluoroethane was replaced by SF₆. Good molten magnesiumprotection was not observed, and the magnesium sample burned rapidly.Adequate protection of the molten magnesium sample was only achievedwhen the gaseous composition consisted of 0.05% by volume SF₆ and thebalance dry air. At this concentration of SF₆ deliberate rupturing ofthe surface film resulted in localised burning of the molten magnesiumsample.

Example 1 and Comparative Example 1 demonstrate that the inventive covergas composition provides good protection of molten magnesium at a lowerconcentration than an SF₆ based composition.

Example 2

A series of single ingots of both pure magnesium and magnesium-aluminiumalloy AZ91 were cast in an 8 kg ingot mould within a controllableatmosphere chamber. The molten metal was sucked under vacuum into thechamber to fill the ingot mould. When the ingot mould was full, thevacuum was turned off, the chamber was filled with a cover gascomposition, and the molten metal was allowed to solidify. In the caseof AZ91 alloy the cover gas composition consisted of 0.04% by volume1,1,1,2-tetrafluoroethane and the balance dry air. The cover gascomposition for the pure magnesium casting consisted of 0.1% by volume1,1,1,2-tetrafluoroethane and the balance dry air.

Single ingots of both pure magnesium and AZ91 alloy were produced freeof burning, with bright shiny surface finishes, with very low levels ofdross, and with no reaction with boron nitride mould coatings.

Comparative Example 2

Comparative Example 2 was identical to Example 2 with the exception that1,1,1,2-tetrafluoroethane was replaced by SF which was used at the sameconcentrations, ie. 0.04% by volume in dry air for AZ91 alloy and 0.1%by volume in dry air for pure magnesium.

The ingots produced in Example 2 had lower levels of dross and had amore attractive surface finish than those produced in ComparativeExample 2.

Example 3

A small flow of 1,1,1,2-tetrafluoroethane was continuously metered intoa container that is used to collect molten magnesium dross. Duringtransport of the dross from the furnace to the container, the drosscontacted the air and ignited. Upon placing the dross into thecontainer, the burning quickly stopped.

Comparative Example 3

Comparative Example 3 was identical to Example 3 with the exception that1,1,1,2-tetrafluoroethane was replaced by SF₆. In this case, the drosscontinued to burn after being placed into the container.

Example 3 and Comparative Example 3 demonstrate that an inhibiting agentof the present invention is able to suppress the burning of magnesiummetal/dross. This enables minimisation of magnesium fume in a workingenvironment and prevention of oxidation of the magnesium metal contentin the dross. This would enable dross processing operations to recovervaluable magnesium metal content.

Example 4

Ingots of pure magnesium were cast in 8 kg ingot moulds on anindustrial-sized ingot casting machine having a controllable atmospherechamber. The casting machine was operated at a casting rate of 3 tonnesof cast metal per hour with 330 liters per minute dry air and 3.3 litersper minute 1,1,1,2-tetrafluoroethane introduced into the chamber. Ingotswere produced free of burning, with bright shiny surface finishes, withvery low levels of dross and with no reaction with boron nitride mouldcoatings.

Comparative Example 4

Comparative Example 4 was identical to Example 4 with the exception that1,1,1,2-tetrafluoroethane was replaced by SF₆ which was used at the sameflow rate and at the same concentration in dry air. Ingots produced inComparative Example 4 exhibited similar properties to those produced inExample 4.

Example 4 and Comparative Example 4 demonstrate that the inventive gascan successfully replace SF₆ for industrial scale continuous productionof magnesium ingot.

Example 5

A series of single ingots of pure magnesium were cast in an 8 kg ingotmould within a controllable atmosphere chamber. The molten metal wassucked under vacuum into the chamber to fill the ingot mould. When theingot mould was full, the vacuum was turned off, the chamber was filledwith cover gas composition, and the molten metal was allowed tosolidify. The cover gas composition was produced by passing 0.5 litersper minute of dry air over 50 ml of the HFE liquidmethoxy-nonafluorobutane. The resulting gas phase mixture flowed to thesingle ingot casting apparatus. Single ingots were produced free ofburning, with bright shiny surface finishes, with very low levels ofdross and with no reaction with boron nitride mould coatings.

Example 6

A series of single ingots of pure magnesium were cast in an 8 kg ingotmould within a controllable atmosphere chamber. The molten metal wassucked under vacuum into the chamber to fill the ingot mould. When theingot mould was full, the vacuum was turned off, the chamber was filledwith a cover gas composition, and the molten metal was allowed tosolidify. The cover gas composition was produced by passing 0.5 litersper minute of dry air over 50 ml of the HFC liquiddihydrodecafluoropentane. The resulting gas phase mixture flowed to thesingle ingot casting apparatus. Single ingots were produced free ofburning, with bright shiny surface finishes, with very low levels ofdross and with no reaction with boron nitride mould coatings.

Example 7

A furnace containing 20 kg of molten magnesium at 700° C. was blanketedwith a cover gas composition. The cover gas composition was produced bypassing 0.6 liters per minute of dry air over 50 ml of the HFE liquidmethoxy-nonafluorobutane. The resulting gas phase mixture flowed to thefurnace. Good molten magnesium protection was observed, with theformation of a thin protective surface film. Deliberate rupturing of thesurface film did not induce burning of the molten magnesium sample.

Example 8

A furnace containing 20 kg of molten magnesium at 700° C. was blanketedwith a cover gas composition. The cover gas composition was produced bypassing 0.9 liters per minute of dry air over 50 ml of the HFE liquidethoxy-nonafluorobutane. The resulting gas phase mixture flowed to thefurnace. Good molten magnesium protection was observed, with theformation of a thin protective surface film. Deliberate rupturing of thesurface film did not induce burning of the molten magnesium sample.

Example 9

A furnace containing 20 kg of molten magnesium at 700° C. was blanketedwith a cover gas composition. The cover gas composition was produced bypassing 0.9 liters per minute of dry air over 50 ml of the HFC liquiddihydrodecafluoropentane. The resulting gas phase mixture flowed to thefurnace. Good molten magnesium protection was observed, with theformation of a thin protective surface film. Deliberate rupturing of thesurface film did not induce burning of the molten magnesium sample.

Example 10

A furnace containing 20 kg of molten magnesium at 700° C. was blanketedwith a gaseous composition consisting of 0.4% by volume difluoroethaneand the balance dry air. Good molten magnesium protection was observed,with the formation of a thin protective surface film. Deliberaterupturing of the surface film did not induce burning of the moltenmagnesium sample.

Comparative Example 10

Comparative Example 10 was identical to Example 10 with the exceptionthat difluoroethane was replaced by SF₆ which was used at the sameconcentration. Good molten magnesium protection was observed.

Example 10 and Comparative Example 10 demonstrate that an inhibitingagent of the present invention provides equivalent protection of moltenmagnesium metal compared to SF₆.

Example 11

Magnesium squeeze-castings were produced by hand-pouring moltenmagnesium into the shot sleeve of a vertical injection squeeze castingmachine. Prior to pouring the molten magnesium into the shot sleeve, asmall volume of pure 1,1,1,2-tetrafluoroethane was introduced into theshot sleeve. This protected the molten magnesium in the shot sleeve andprevented the molten magnesium from burning during the filling of themould.

Example 12

Various magnesium components were produced using the investment castingtechnique. Prior to filling the investment casting shell with moltenmagnesium, the shell was purged with pure 1,1,1,2-tetrafluoroethane.This prevented the magnesium from burning while solidifying inside theshell. Upon cooling, the shell mould was removed. The magnesium castingexhibited a good surface finish.

Example 13

Various magnesium components were produced using the sand castingtechnique. Prior to filling the sand mould with molten magnesium, thesand mould was purged with pure 1,1,1,2-tetrafluoroethane. Thisprevented the magnesium from burning while solidifying inside the sandmould. Upon cooling, the sand mould was removed. The magnesium castingexhibited a good surface finish.

Example 14

A melt furnace having a diameter of 1.6 meters and containing 4 tonnesof molten pure magnesium was blanketed with 60 liters per minute dry airand 0.6 liters per minute 1,1,1,2-tetrafluoroethane. Good moltenmagnesium protection was observed, with the formation of a thinprotective surface film.

Comparative Example 14

Comparative Example 14 was identical to Example 14 with the exceptionthat 1,1,1,2-tetrafluorethane was replaced by SF₆ at differing flowrates. The flow rate of dry air was maintained at 60 liters per minute.Good molten magnesium protection was only achieved at an SF₆ flow rateof 2 liters per minute.

Example 14 and Comparative Example 14 demonstrate that the inventivecover gas composition provides good industrial scale protection ofmolten magnesium at a lower concentration than an SF₆ based composition.

1. A cover gas composition for the protection of moltenmagnesium/magnesium alloy from oxidation, consisting of a fluorinecontaining inhibiting agent in an effective amount to inhibit theoxidation of the molten magnesium/magnesium alloy, selected from thegroup consisting of difluoromethane, pentafluoroethane,1,1,1,2-tetrafluoroethane, difluoroethane, heptafluoropropane,dihydrodecafluoropentane, hydrofluoroethers and mixtures thereof, and acarrier gas, wherein each component of the composition has a GlobalWarming Potential (GWP) (referenced to the absolute GWP for carbondioxide at a time horizon of 100 years) of less than
 5000. 2. Acomposition as claimed in claim 1, wherein the inhibiting agent has noozone depletion potential.
 3. A composition as claimed in claim 1,wherein the carrier gas is selected from the group consisting of air,carbon dioxide, argon, nitrogen and mixtures thereof.
 4. A compositionas claimed in claim 1, wherein each component of the composition has GWPof less than
 3000. 5. The composition of claim 1, wherein the amount offluorine containing inhibiting agent in the cover gas composition variesfrom the minimum amount effective to inhibit the oxidation of the moltenmagnesium/magnesium alloy up to less than 1% by volume of the cover gascomposition.
 6. A composition as claimed in claim 1 wherein theinhibiting agent has a boiling point of less than 100° C.
 7. Acomposition as claimed in claim 1, wherein the hydrofluoroethers areselected from the group consisting of methoxy-nonafluorobutane,ethoxy-nonafluorobutane, and mixtures thereof.
 8. A composition asclaimed in claim 4 wherein each component of the composition has a GWPof less than
 1500. 9. A composition as claimed in claim 7 wherein theinhibiting agent is 1,1,1,2-tetrafluoroethane and the carrier gas is dryair.
 10. A composition as claimed in claim 1, wherein the inhibitingagent is selected from the group consisting of difluoromethane,pentafluoroethane, 1,1,1,2-tetrafluoroethane, difluoroethane,heptafluoropropane, dihydrodecafluoropentane, and mixtures thereof. 11.A composition as claimed in claim 1 containing up to less than 0.5% byvolume inhibiting agent.
 12. A composition as claimed in claim 11,containing up to less than 0.1% by volume inhibiting agent.
 13. Acomposition as claimed in claim 7, wherein the inhibiting agent is1,1,1,2-tetrafluoroethane and the carrier gas is selected from the groupconsisting of nitrogen, carbon dioxide and mixtures thereof.
 14. Amethod of protecting molten magnesium/magnesium alloy from oxidation,comprising blanketing the magnesium/magnesium alloy with a cover gascomposition consisting of a fluorine containing inhibiting agent in aneffective amount to inhibit the oxidation of the moltenmagnesium/magnesium alloy, selected from the group consisting ofdifluoromethane, pentafluoroethane, 1,1,1,2-tetrafluoroethane,difluoroethane, heptafluoropropane, dihydrodecafluoropentane,hydrofluoroethers, and mixtures thereof, and a carrier gas, wherein eachcomponent of the composition has a Global Warming Potential (GWP)(referenced to the absolute GWP for carbon dioxide at a time horizon of100 years) of less than
 5000. 15. A method as claimed in claim 14,wherein the inhibiting agent is 1,1,1,2-tetrafluoroethane.
 16. A methodas claimed in claim 14, wherein said fluorine containing inhibitingagent is a hydrofluoroether.
 17. A method as claimed in claim 16,wherein said hydrofluoroether is selected from the group consisting ofmethoxynonafluorobutane, ethoxynonafluorobutane, and mixtures thereof.18. A method as claimed in claim 14, wherein said carrier gas isselected from the group consisting of air, CO₂, argon, nitrogen, andmixtures thereof.
 19. The method of claim 14, wherein the amount offluorine containing inhibiting agent in the cover gas composition variesfrom the minimum amount effective to inhibit the oxidation of the moltenmagnesium/magnesium alloy up to less than 1% by volume of the cover gascomposition.
 20. A method as claimed in claim 14, wherein the inhibitingagent is selected from the group consisting of difluoromethane,pentafluoroethane, 1,1,1,2-tetrafluoroethane, difluoroethane,heptafluoropropane, dihydrodecafluoropentane, and mixtures thereof. 21.A method for protecting an exposed surface of molten magnesium/magnesiumalloy from oxidation in ambient air, comprising: contacting the exposedsurface of the molten magnesium/magnesium alloy with a gaseous mixtureconsisting of a fluorine containing inhibiting agent in an effectiveamount to inhibit the oxidation of the molten magnesium/magnesium alloy,selected from the group consisting of difluoromethane,pentafluoroethane, 1,1,1,2-tetrafluoroethane, difluoroethane,heptafluoropropane, dihydrodecafluoropentane, hydrofluoroethers, andmixtures thereof, and a carrier gas to form a protective film/layer onthe surface of said molten magnesium/magnesium alloy.
 22. A method asclaimed in claim 21, wherein said carrier gas is selected from the groupconsisting of air, CO₂, argon, nitrogen, and mixtures thereof.
 23. Amethod as claimed in claim 21, wherein the inhibiting agent is1,1,1,2-tetrafluoroethane.
 24. A method as claimed in claim 21, whereinsaid fluorine containing inhibiting agent is a hydrofluoroether.
 25. Amethod as claimed in claim 24, wherein said hydrofluoroether is selectedfrom the group consisting of methoxynonafluorobutane,ethoxynonafluorobutane, and mixtures thereof.
 26. The method of claim21, wherein the amount of fluorine containing inhibiting agent in thecover gas composition varies from the minimum amount effective toinhibit the oxidation of the molten magnesium/magnesium alloy up to lessthan 1% by volume of the cover gas composition.
 27. A method as claimedin claim 21, wherein the inhibiting agent is selected from the groupconsisting of difluoromethane, pentafluoroethane,1,1,1,2-tetrafluoroethane, difluoroethane, heptafluoropropane,dihydrodecafluoropentane, and mixtures thereof.